1. Field of the Invention
The present invention relates to a cutting insert and a cutting tool in which the cutting insert is mounted. In particular, the present invention relates to a tangential cutting insert for use in grooving (slotting) and an indexable rotary cutting tool in which the cutting insert is detachably mounted.
2. Description of the Related Art
An indexable side cutter is a conventional rotary cutting tool which can carry out grooving. In general, the grooving needs to provide a round corner formed of a curved surface, that is, a curved corner surface, in a corner between a groove bottom processed in a workpiece and a side wall surface continuous with the groove bottom. Stress is likely to concentrate between the groove bottom and the side wall surface. Thus, without the curved corner surface, the workpiece may be cracked starting from the corner portion and may thus be likely to be damaged. Consequently, it is important for the indexable side cutter to include a cutting insert which allows round chamfering for various sizes, that is, processing for forming a curved surface, to be carried out on groove corners.
A conventional cutting insert is shaped, for example, as shown in FIG. 18. A cutting insert 601 in FIG. 18 is of a tangential type which appears shaped substantially like a rectangle in a plan view. The insert 601 includes cutting edges 605 formed at edges of a set of opposite side surfaces 603, that is, the side surfaces 603 positioned opposite each other, and round corner surfaces, that is, curved corner surfaces 607, formed to extend between the opposite side surfaces. The corners of the cutting edges 605 extending along the edges of the round corner surfaces 607 can serve to form curved corner surfaces at corners of a groove.
Furthermore, Japanese Patent Laid-Open No. H6-262422 (1994) discloses another form of a tangential cutting insert. FIG. 19 shows an example of a conventional cutting insert with cutting edges provided at positions similar to those on the cutting insert in Japanese Patent Laid-Open No. H6-262422 (1994). A cutting insert 701 in FIG. 19 appears shaped substantially like a rectangle in a plan view. The cutting insert 701 includes cutting edges 703 arranged at equal intervals in a circumferential direction around an insert central axis. The cutting insert 701 is no-hand insert, and all the cutting edges 703 are oriented in the respective predetermined directions. A cutting insert of this type may further include such round corner surfaces as shown in FIG. 18 in order to process curved corner surfaces at corners of a groove.
In general, when a cutting insert is mounted in a tool body of a cutting tool, the cutting insert is preferably mounted so as to have a radial rake angle of, for example, 0° or a positive radial rake angle. Alternatively, even if the radial rake angle is negative, the cutting insert is preferably mounted in the tool body so as to reduce the level of the negativity. A reason for this is that a larger negative radial rake angle enhances impact and cutting resistance resulting from cutting of a cutting edge into a workpiece, thus increasing the possibility of damaging the cutting insert.
For the conventional cutting insert shown in FIG. 18, the length between the side surfaces of the cutting insert needs to be reduced in order to arrange the cutting insert such that the cutting insert has a small negative radial rake angle or a positive radial rake angle around the tool body of the rotary cutting tool. However, the length between the side surfaces of the cutting insert, that is, the length of the round corner surface, is equal to the diameter of an inscribed circle of the cutting insert as seen in a plan view. That is, the length between the side surfaces is in proportion to the size of the cutting insert as a whole. Thus, reducing the length between the side surfaces needs to decrease the size of the cutting insert as a whole. However, the cutting insert needs to include a mounting hole for attaching and detaching of the cutting insert to a tool body, formed in the center thereof and having a given size. Hence, a simple reduction in the size of the cutting insert may disadvantageously significantly decrease the strength of the cutting insert. Furthermore, a simple increase in the size of the round corner surface correspondingly reduces the contact area between an abutment surface of an insert mounting seat in the tool body and the side surfaces of the cutting insert. This may disadvantageously hinder the cutting insert from being appropriately fixed.
Furthermore, like the cutting insert in FIG. 18, such a cutting insert as shown in Japanese Patent Laid-Open No. H6-262422 (1994) or FIG. 19 has a problem with the radial rake angle. Moreover, when such round corner surfaces as described above are formed on a cutting insert of the type shown in FIG. 19, the design of the round corner surfaces is limited because the cutting edges are formed at intervals of 90° around the central axis of the mounting hole. If the round corner surfaces formed have a certain size or larger, the round corner surface for one cutting edge may affect the adjacent cutting edge. This hinders the cutting edges from being formed at intervals of 90°, thus possibly reducing the number of cutting edges in the cutting insert. Curved corner surfaces of a groove to be formed vary depending on the width and depth of the groove to be processed. Thus, the lack of the ability to freely design round corner surfaces corresponding to curved corner surfaces of various sizes is very disadvantageous.